(a) Field of the Invention
This invention relates to an electroluminescent panel for use in an input/output device of a computer or the like to display an image, such as alphanumeric symbols, a static picture, a motion picture, and the like.
(b) Description of Prior Art
As a rule, an electroluminescent panel of the type described comprises a transparent substrate, a transparent or front electrode on the substrate, a back electrode opposite the transparent electrode, and an electroluminescent layer between the front and back electrodes.
As will later be described with reference to a few figures of the accompanying drawing, an electroluminescent panel is proposed by John M Lo in GB 2,039,146A and comprises a light absorption layer between the electroluminescent layer and the back electrode. According to Lo, the light absorption layer must preferably have a resistivity between 10.sup.2 .OMEGA..multidot.cm and 10.sup.5 .OMEGA..multidot.cm so as to improve contrast and brightness. To this end, Lo's attention is mainly directed to telluride materials, such as cadmium telluride (CdTe), zinc telluride (ZnTe), lead telluride (PbTe), and tin telluride (SnTe).
However, it has been found that a desired brightness can be accomplished only by supply of a comparatively high electric voltage between the front and back electrodes because the brightness slowly or gently increases with an increase of the electric voltage. Such supply of a high electric voltage might give rise to dielectric breakdown in the electroluminescent panel.
In Japanese Unexamined Patent Publication No. Sy o 59-154,793, namely, 154,793/1984, an improved electroluminescent panel is disclosed which comprises amorphous semiconductor layers between the front electrode and the electroluminescent layer and between the electroluminescent layer and the back electrode. Each amorphous semiconductor layer is of silicon or silicon carbide (SiC) and has therefore a comparatively high resistivity, as known in the art. Thus, each amorphous layer has a high resistivity.
According to the inventor's experimental studies, intervention of such a high resistivity layer results in an increase of the electric voltage supplied between the front and back electrodes.
In U.S. patent application Ser. No. 728,595 assigned to the instant assignee, S. Kawai discloses another electroluminescent panel comprising a light absorption layer of a lower-order oxide of germanium. With this electroluminescent panel, it is possible to accomplish a desired brightness by supply of a comparatively low electric voltage.
However, the lower-order oxide of germanium is poor in chemical proof and is susceptible to aging in an electric field resulting from supply of the electric voltage. Accordingly, the electroluminescent panel is low in yield on fabrication thereof and has a short life in practical use.